1. Field of the Invention
The present invention relates to an optical receiver which detects signal light with an avalanche photodiode.
2. Description of the Background Art
In an optical communication system, an optical receiver employing an avalanche photodiode (hereafter abbreviated as APD) as a photodetector is used conventionally. APDs have a function to amplify signal photocurrents and are suitable for use as photodetectors in optical fiber communication systems using weak optical signals.
A typical APD has a characteristic that an avalanche multiplication factor thereof varies due to temperature variation or the like. This is due to a relatively high reverse bias voltage applied to the APD when avalanche multiplication is performed. More specifically, the APD is operated at a voltage close to a break-down voltage of a PN junction, and therefore the operating characteristics of the APD are extremely sensitive to variation in ambient temperature or the like. Accordingly, the avalanche multiplication factor of the APD is preferably controlled such that a constant multiplication factor can be obtained even when temperature variation or the like occurs.
Japanese Unexamined Patent Application Publication No. 63-77171 discloses an optical receiver having a structure for controlling a multiplication factor of an APD. The structure of this optical receiver is shown in FIG. 10(a). This optical receiver has an optical-signal-receiving area 101 and an optical-signal-monitoring area 102 on a common substrate, and receives signal light from an optical fiber 103 at these areas. An avalanche photodiode is provided in the optical-signal-receiving area 101, and a multiplication factor at the optical-signal-receiving area 101 is controlled on the basis of an output current value obtained at the optical-signal-monitoring area 102.
In addition, Japanese Unexamined Patent Application Publication No. 63-105541 discloses another optical receiver having a different structure for controlling a multiplication factor of an APD. The structure of this optical receiver is shown in FIG. 10(b). This optical receiver includes an APD 111 and a photodiode 112. The APD 111 receives signal light emitted from an optical fiber 113, and the photodiode 112 receives signal light reflected by a light-receiving area of the APD 111. The multiplication factor of the APD 111 is controlled on the basis of an output current value obtained by the photodiode 112.
In the optical receiver disclosed in Japanese Unexamined Patent Application Publication No. 63-77171, the optical-signal-receiving area 101 and the optical-signal-monitoring area 102 are formed adjacently on the same plane of the same substrate. Therefore, it is highly likely that crosstalk will occur between the optical-signal-receiving area 101 and the optical-signal-monitoring area 102. When the crosstalk occurs, the accuracy of detection of the signal light at the optical-signal-monitoring area 102 is reduced and it is therefore difficult to control the multiplication factor at the optical-signal-receiving area 101 with high accuracy.
In the optical receiver disclosed in Japanese Unexamined Patent Application Publication No. 63-105541, the signal light is incident on the APD 111 inevitably at an angle. However, when the signal light is incident on the light-receiving area of the APD 111 at an angle, the reflectance of the signal light varies depending on the polarization state of the signal light. Although the dependency on the polarization state can be eliminated by forming a multilayer film, it is difficult to form the multilayer film on the light-receiving area of the APD 111, which is a semiconductor device. Therefore, in this optical receiver, the quantity of signal light incident on the photodiode 112 varies depending on the polarization state of the signal light emitted from the optical fiber 113, and it is difficult to control the avalanche multiplication factor of the APD 111 with high accuracy.